a) Field of the Invention
The invention relates to a pivot device for a suspended cutting knife of a cutting head of an automatically controlled cutting machine for cutting fabric sheet material spread out on a cutting table in multiple layers, which cutting head is controlled according to a three-dimensional coordinate system by means for moving the cutting head along the X- and Y-axis and pivoting about the Z-axis for moving the knife tangently along a predetermined cutting path during cutting of the material. The cutting knife is mounted so as to be reciprocally movable along the Z-axis. The cutting head further comprises guiding means for guiding the unsuspended part of the cutting knife in the cutting head and a pressure foot rigidly connected to the pivotable cutting head.
b) Description of the Related Art
Automatically closed loop controlled cutting machines for cutting sheet material as fabrics for garments spread out on the cutting table in multiple layers being held onto the cutting table by atmospheric pressure are well known.
One of the problems of such cutting machines is that without corrective measures the knife will track a cutting path in the upper ply of the layup slightly different from the cutting path in the lower ply so that the pattern pieces from the respective plies will have slightly different shapes. Therefore the height of the staple of layers to be cut is limited by the knife bending stiffness for a desired cutting quality.
Known means for compensating for defects depending on bending flexure of the knife of an automatic cutting machine comprises sensors for sensing the lateral forces acting on the flanks of the knife during cutting. These signals are transferred and applied to a computer or processor which provides correcting signals representing an additional angle or correction angles being superimposed to the orientation of the preprogrammed cutting path of the knife around the Z-axis with respect to its path; see U.S. Pat. No. 4,133,235.
According to GB-2 094 031, digital sensors are used for detecting the bending of the knife and providing signals indicating the presence of flexure and its direction. By feeding back knife position to a servomechanism, the required correction is computed in conjunction with these signals.
According to both known methods, the required correction of the knife angle has to be computed in conjunction with lateral force signals and information concerning the properties of the material to be cut in order to minimize defects depending on knife flexure.
Therefore such methods require that lateral forces acting on the knife are correctly measured and transformed into correcting signals to modify the preprogrammed orientation of the knife around its longitudinal or Z-axis and require further a relatively great expenditure in sensors, transducers, actuators and in data logger feedback gauging systems which are very complex and thus quite expensive and are further difficult to handle.
According to experience, loads acting on the knife during cutting operation are of different types; one of these are lateral loads effecting knife bending. These lateral loads acting onto the flanks of the knife are caused by the pressure of the fabric to be cut during interaction of the cutting knife and sheet material, which generates friction loads in the feeding direction of the moving knife also. The pressure of the fabric to be cut can be different at both sides of the knife due to different reasons, such as the anisotropy of the fabrics or the proximity of a previous cut or the fabric border at one side of the knife.
The relations between lateral pressure and knife bending without evaluating other dependencies are generally indicated in FIGS. 2 and 3. Under the adoption that the pressure on a point of the knife is proportional to the compression of the fabric at this point, the following correlations are applicable.
FIG. 2 shows a sectional view on a staple of layers whereby line "t" is the theoretic path which is the path followed by the knife without bending whereas line "r" is the actual path in the section due to knife bending. Deviation in this section is "d", thus the pressure can be expressed by EQU p=K.multidot.y (1)
"K" being a constant that, in general, can be different at each side of the knife due to the anisotropy or the proximity to a previous cut line, as mentioned above.
FIG. 3 shows the assumption that EQU p'=K'.multidot.y&gt;p=K.multidot.y (2)
If the knife could pivot in relation to its path around an axis near its leading edge, the pressure appearing at each flank of the knife will change according to the distances "y" of every point to the cut line as shown in FIG. 3. The rotation about this axis in front of the knife would tend to balance the lateral loads, which leads to a decreasing or avoiding of the bending on the knife and the deviation "d".